1. Field of the Invention
The invention is concerned with the fabrication of optical fibers.
2. Description of the Prior Art
A promising approach to the fabrication of optical waveguides on an industrial scale involves the preparation of a preform in the form of a glass rod, followed by drawing a fiber from the rod.
The perform may consist of a single, homogeneous glass, it may consist of two glasses of which one forms a core and the other a cladding, or it may consist of several coaxial cylindrical layers of glasses. As compared with its preform, a drawn fiber has greatly reduced diameter but identical compositional structure.
Effective light guiding in an optical fiber requires a structure in which an optically transparent core material is surrounded by another material whose refractive index is less than that of the core material. Such structure may be achieved by coating or sheathing a drawn fiber or by an appropriate choice of preform core and cladding glasses. For example, pure silica may be used as a cladding glass in combination with germanium oxide, phosphorous oxide or titanium oxide doped silica as a core glass. Or, pure silica may be used as a core glass in combination with boron doped silica as a cladding glass. Other glass compositions such as sodium borosilicates and sodium lime silicates have also been studied in this context.
While fibers composed of one or two glasses are well suited for single-mode transmission, fibers for multimode transmission are preferably made of a great number of coaxial glass layers whose refractive index varies essentially continuously and preferably according to an optimal grading. Either type of fiber may be made by drawing techniques described in the paper by William G. French, John B. MacChesney, and A. David Pearson, on "Glass Fibers for Optical Communications" which appeared in the Annual Review of Materials Science, Vol. 5, 1975, pages 373-394.
For the manufacture of a preform a number of methods have been proposed. One method, known as "soot process" is based on the deposition of microscopic glass particles on a substrate at relatively low temperatures, followed by heat fusing the resulting layer of particles. For example, a mixture of SiCl.sub.4 and TiCl.sub.4 vapors may be hydrolized in a gas-oxygen burner to deposit glass particles on the inside of a silica tube. The particles are subsequently heat fused and the resulting assembly is collapsed into a rod-shaped preform having a titanium-doped silica core and a pure silica cladding. The soot process may also be used to build up glass deposits on the outside of a tube or solid mandrel which, after deposition and fusing, may be removed if required, by chemical etching or mechanical core drilling, respectively.
Alternate methods for making a preform are disclosed in U.S. patent applications Ser. No. 444,705, filed on Oct. 23, 1973, and Ser. No. 670,162, filed on Mar. 25, 1976, both now abandoned. Application Ser. No. 444,705, by J. B. MacChesney and P. B. O'Connor, and entitled "Optical Fiber Fabrication and Resulting Product" calls for directing a flow of a gaseous mixture of reagents into a tube which is heated externally by a moving hot zone. Glass particles produced as a result of a homogeneous chemical reaction in the gaseous mixture as well as a glass formed directly on the substrate by heterogeneous reaction are deposited on the inside of the tube and are fused into a continuous layer of glass. Application Ser. No. 670,162 by R. E. Jaeger, J. B. MacChesney, and T. J. Miller, and entitled "Modified Chemical Vapor Deposition of an Optical Fiber Using an rf Plasma", calls for directing a flow of a gaseous mixture of reagents into a tube and depositing glass on the inside of the tube as a result of a chemical reaction taking place in a radio frequency plasma. A method has also been proposed in which a chemical reaction takes place in a microwave frequency plasma.
One feature shared by these methods is the need for precisely proportioned mixtures of gaseous reagents. While it has been possible to produce such mixtures by carefully regulating flows of reagent gases under laboratory conditions, difficulties have arisen in adapting this approach to the production of highly uniform fibers in an industrial setting. These difficulties may be compared with those encountered in the manufacture of GaAs Schottky barrier diodes where it is necessary to supply precisely controlled bursts of dopant reagents to chemical vapor deposition apparatus. U.S. Pat. No. 3,904,449 issued on Sept. 9, 1975 to J. V. DiLorenzo and L. C. Luther and assigned to the present assignee, discloses a method which achieves such control by injecting into a carrier gas a predetermined volume of a gaseous dopant reagent at predetermined pressure.